As a result of environmental and other concerns, plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers are now being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Blow-molded plastic containers have become commonplace in packaging numerous commodities. Studies have indicated that the configureation and overall aesthetic appearance of a blow-molded plastic container can affect consumer purchasing decisions. For example, a dented, distorted or otherwise unaesthetically pleasing container may provide the reason for some consumers to purchase a different brand of product which is packaged in a more aesthetically pleasing fashion.
While a container in its as-designed configuration may provide an appealing appearance when it is initially removed from a blow-molding machine, many forces act subsequently on, and alter, the as-designed shape from the time it is blow-molded to the time it is placed on a store shelf. Plastic containers are particularly susceptible to distortion since they are continually being re-designed in an effort to reduce the amount of plastic required to make the container. While this strategy realizes a savings with respect to material costs, the reduction in the amount of plastic can decrease container rigidity and structural integrity.
Manufacturers currently supply PET containers for various liquid commodities, such as juice and isotonic beverages. Suppliers often fill these liquid products into the containers while the liquid product is at an elevated temperature, typically between 155° F.-205° F. (68° C.-96° C.) and usually at approximately 185° F. (85° C.). When packaged in this manner, the hot temperature of the liquid commodity sterilizes the container at the time of filling. The bottling industry refers to this process as hot filling, and the containers designed to withstand the process as hot-fill or heat-set containers.
The hot filling process is acceptable for commodities having a high acid content, but not generally acceptable for non-high acid content commodities. Nonetheless, manufacturers and fillers of non-high acid content commodities desire to supply their commodities in PET containers as well.
For non-high acid content commodities, pasteurization and retort are the preferred sterilization processes. Pasteurization and retort both present an enormous challenge for manufactures of PET containers in that heat-set containers cannot withstand the temperature and time demands required of pasteurization and retort.
Pasteurization and retort are both processes for cooking or sterilizing the contents of a container after filling. Both processes include the heating of the contents of the container to a specified temperature, usually above approximately 155° F. (approximately 70° C.), for a specified length of time (20-60 minutes). Retort differs from pasteurization in that retort uses higher temperatures to sterilize the container and cook its contents. Retort also applies elevated air pressure externally to the container to counteract pressure inside the container. The pressure applied externally to the container is necessary because a hot water bath is often used and the overpressure keeps the water, as well as the liquid in the contents of the container, in liquid form, above their respective boiling point temperatures.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:
      %    ⁢                  ⁢    Crystallinity    =            (                        ρ          -                      ρ            a                                                ρ            c                    -                      ρ            a                              )        ⨯    100  where ρ is the density of the PET material; ρa is the density of pure amorphous PET material (1.333 g/cc); and ρc is the density of pure crystalline material (1.455 g/cc).
Container manufacturers use mechanical processing and thermal processing to increase the PET polymer crystallinity of a container. Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching a PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what manufacturers define as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having approximately 20% crystallinity in the container's sidewall.
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque, and thus, generally undesirable. Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity for those portions of the container having biaxial molecular orientation. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heated to a temperature of approximately 250° F.-350° F. (approximately 121° C.-177° C.), and holding the blown container against the heated mold for approximately two (2) to five (5) seconds. Manufacturers of PET juice bottles, which must be hot-filled at approximately 185° F. (85° C.), currently use heat setting to produce PET bottles having an overall crystallinity in the range of approximately 25% -30%.
After being hot-filled, the heat-set containers are capped and allowed to reside at generally the filling temperature for approximately five (5) minutes at which point the container, along with the product, is then actively cooled prior to transferring to labeling, packaging, and shipping operations. The cooling reduces the volume of the liquid in the container. This product shrinkage phenomenon results in the creation of a vacuum within the container. Generally, vacuum pressures within the container range from 1-380 mm Hg less than atmospheric pressure (i.e., 759 mm Hg-380 mm Hg). If not controlled or otherwise accommodated, these vacuum pressures result in deformation of the container, which leads to either an aesthetically unacceptable container or one that is unstable. Hot-fillable plastic containers must provide sufficient flexure to compensate for the changes of pressure and temperature, while maintaining structural integrity and aesthetic appearance. Typically, the industry accommodates vacuum related pressures with sidewall structures or vacuum panels formed within the sidewall of the container. Such vacuum panels generally distort inwardly under vacuum pressures in a controlled manner to eliminate undesirable deformation.
While vacuum panels allow containers to withstand the rigors of a hot-fill procedure, the panels have limitations and drawbacks. First, vacuum panels formed within the sidewall of a container do not create a generally smooth glass-like appearance. Second, packagers often apply a wrap-around or sleeve label to the container over the vacuum panels. The appearance of these labels over the sidewall and vacuum panels is such that the label often becomes wrinkled and not smooth. Additionally, one grasping the container generally feels the vacuum panels beneath the label and often pushes the label into various panel crevasses and recesses.
These traditional containers were not easy for consumers to handle while carrying or dispensing product from the container. Further refinements have led to the use of pinch grip geometry in the sidewall of the containers to help control container distortion resulting from vacuum pressures. However, similar limitations and drawbacks exist with pinch grip geometry as with vacuum panels.
In many instances, container weight is correlated to the amount of the final vacuum present in the container after this fill, cap and cool down procedure, that is, the container is made relatively heavy to accommodate vacuum related forces. Similarly, reducing container weight, i.e., “lightweight” the container, while providing a significant cost savings from a material standpoint, requires a reduction in the amount of the final vacuum.
External forces are applied to sealed containers as they are packed and shipped. Filled containers are packed in bulk in cardboard boxes, or plastic wrap, or both. A bottom row of packed, filled containers may support several upper tiers of filled containers, and potentially, several upper boxes of filled containers. Therefore, it is important that the container have a top loading capability which is sufficient to prevent distortion from the intended container shape.
More recently, container manufacturers have begun introducing multi-serve heat-set containers having a generally rectangular horizontal cross-sectional shape. Similar to the prior containers discussed above, these rectangular containers require a majority of the vacuum forces to be absorbed within the sidewall of the container. However, as these somewhat larger containers become increasingly lighter in weight, the weight of the fluid within the container reduces the amount of vacuum forces that the sidewall portion of the container can accommodate. Thus, this combination of lighter weight containers and increased weight of product within the container causes the sidewall portion of the container to sag and results in unwanted deformation in other areas of the container as well.
In an attempt to accommodate for some of the vacuum forces currently not accounted for in the sidewall, the grip area of current rectangular containers is designed to be flexible. This flexibility is detrimental to the consumer during handling, carrying and dispensing of product from the container. This flexibility may cause the container to slip from the consumer's hand or result in an overall insecure feel. Both of which may negatively effect consumer purchasing decisions.
Thus, there is a need for an improved lightweight rectangular container which can accommodate the vacuum pressures which result from hot filling, preventing container sidewall sag, while providing a more secure grip area which instills confidence in the consumer during handling, carrying and dispensing of product from the container.